This invention relates to a varistor, and more particularly to a varistor having a thick film formed on a substrate and a method of making the same. A varistor is a voltage variable resistor, and its electrical behavior is commonly described by the following characteristic relationship: EQU I = (V/C).sup..alpha.
wherein:
I = current flowing through the varistor; PA1 V = voltage across the varistor; PA1 C = constant; PA1 .alpha. = constant &gt; 1; measure of the non-linearity of the varistor.
There are a number of varistors known in the art in a discrete component form, and they commonly include varistor materials formulated with zinc oxide. In U.S. Pat. No. 3,496,512, granted on Feb. 17, 1970, to Matsuoka, et al., a varistor is disclosed which includes a sintered body of zinc oxide with silver paint electrodes applied to opposite surfaces. The properties are dependent upon the bulk of the device; i.e., the non-linearity is determined to a considerable extent by the composition of the sintered body, and the value of C is controlled by the dimension of the body between the electrodes. There are many other devices of a similar construction in which the sintered body includes together with zinc oxide various metal oxides to effectuate an increase in the non-linearity property. See, for example, U.S. Pat. No. 3,632,528 issued on Jan. 4, 1972, to Matsuoka, et al., for "Lead-Modified Zinc Oxide Voltage Variable Resistor"; U.S. Pat. No. 3,634,337 issued on Jan. 11, 1972, to Matsuoka, et al., for "Barium-Modified Zinc Oxide Voltage Variable Resistor"; U.S. Pat. No. 3,598,763 issued on Aug. 10, 1971, to Matsuoka, et al., for "Manganese-Modified Zinc Oxide Voltage Variable Resistor"; and U.S. Pat. No. 3,699,058 issued on Oct. 17, 1972, to Matsuoka, et al., for "Uranium-Modified Zinc Oxide Voltage Variable Resistor."
Also, varistors have been introduced in a thick film form for use in integrated circuit applications. The patent issued to Wada, et al., on Apr. 3, 1973, for "Thick Film Varistor and Method For Making The Same," discloses a thick film varistor which is made by mixing a zinc oxide composition with a glass material and a liquid vehicle to form a paste. The paste is applied to a flat substrate and heated to a temperature sufficient to evaporate the liquid vehicle and melt the glass material to bond the zinc oxide composition particles and form a thick film. Suitable electrodes are then applied to the surface of the thick film to provide a varistor having a coplanar configuration, i.e., one in which the electrodes are on the same surface of the film. As an alternative, the paste can be applied to an electrode, which is disposed on a substrate, and heated in a similar manner. A second electrode is then applied to the resulting thick film on its surface opposite the electrode which is in contact with the substrate to provide a varistor having a parallel plate configuration, i.e., one in which the electrodes are on opposite surfaces of the film.
Other thick film varistors have been formed on flat substrates, and these varistors include varistor materials containing oxides other than zinc. U.S. Pat. No. 3,900,432 granted to Marcus, et al., on Aug. 19, 1975, for "Varistor Compositions" discloses thick film varistors and compositions containing doped iron oxides and glass powders. The thick film varistors shown and described in U.S. Pat. No. 3,916,366 issued to Jefferson on Oct. 28, 1975 for "Thick Film Varistor And Method Of Making The Same" have varistor materials including nickel oxide and lithium carbonate.
Although thick film varistors have been made, the art is deficient in not providing a thick film varistor in a discrete component form. Such varistors would be desirable since resistors and capacitors having a film deposited on a substrate in a component form are readily available. For example, a resistor of this type is disclosed in the patent issued to Brandt, et al., U.S. Pat. No. 3,808,575, on Apr. 30, 1974, entitled "Cermet Fixed Resistor with Soldered Leads." In this device, a resistive layer is deposited on a cylindrical substrate with openings at opposite ends. A layer is disposed on each end of the substrate in electrical communication with the resistive layer, and solder coated lead wires are within the substrate openings. A strong union is formed between the lead wires and the resistor body, yet the device is relatively inexpensive and easy to manufacture.
It is against this background that the present invention introduces a varistor formed by thick film processing techniques which has the required varistor characteristics, such as a high non-linearity constant, and is volumetrically similar to other thick film electronic components.